The present invention relates to a communication system in which communication is carried out through a communication medium such as a coaxial cable, an optical cable, a telephone circuit or satellite broadcasting and an apparatus for recording and reproducing a signal communicated through the communication medium and more particularly, to a communication system for encrypted data and an apparatus for selectively recording and reproducing a received analog signal and a received digital signal.
A technique concerning a system for encryption and decryption of a video signal and an audio signal as well as fee charging is described in, for example, xe2x80x9cTransactions The Institute of Television Engineers of Japanxe2x80x9d, Vol. 46, No. 1, January, 1992, pp. 31-39.
The above paper describes a technique concerning the system for encryption and decryption of a video signal and an audio signal as well as fee charging in satellite communication service. However, that paper fails to disclose method and technique which can store a received signal while protecting a copyright of the signal and can charge fees of decryption and storage of the signal.
A technique of selecting a digital signal or an analog signal and recording it on the same magnetic recording and reproducing apparatus is described in, for example, JP-A-5-207507.
Available for a domestic magnetic recording and reproducing apparatus (hereinafter referred to as VTR) are, for example, VHS, xcex2 and 8 millimeter-VTR standards. In any of these standards, an analog television signal is FM modulated and then recorded. It is stipulated that an audio signal can also be FM modulated and recorded in accordance with each standard. An apparatus for multiplexing, recording and reproducing analog video and audio signals in this manner is disclosed in U.S Pat. No. 4,591,924.
When a television signal is FM modulated for analog recording, there arises a problem that the S/N ratio is inevitably deteriorated owing to recording and reproduction. To cope with this problem, the television signal is digitized and then recorded. For example, D-1, D-2 and D-3 standards prescribed by SMPTE are available. These standards are determined by taking the use for broadcasting into account and therefore, a tape cassette different from that used for the aforementioned domestic VTR is used under these standards.
The fact that quality of recording and reproduction is improved by recording and reproducing a digital signal is desirable but when user""s merit is taken into consideration, it is preferable that the conventional analog signal be also recorded and reproduced. In other words, exchangeable reproduction of analog and digital signals must be taken into account.
This requires that at least the shape of a cassette to be used be the same for analog and digital signals and the tape transport system be also the same for analog and digital signals. Further, by making the recording and reproducing head or at least the azimuth angle of the head identical for analog and digital signals, exchangeability can be facilitated and cost reduction can be ensured.
But it has been found that the above expedient faces a new problem. For example, when the digitally recorded cassette is inserted by mistake into the conventional analog VTR, the recorded digital signal is mistaken for an FM signal and is FM demodulated inadvertently. Especially when the FM demodulated signal is an audio signal, the demodulated audio signal has a large amplitude and so large a sound that possibly surprises persons involved and damages the apparatus is generated from a loudspeaker.
The aforementioned conventional technique fails to show what problem arises when the digitally recorded cassette is loaded on the conventional analog VTR. Needless to say, the conventional technique does not clarify how to solve the problem.
An object of the present invention is to provide an apparatus for selectively recording and reproducing an analog signal and a digital signal.
Another object of the present invention is to provide a data receiving unit which can receive and decrypt or store an encrypted signal while protecting a copyright of the signal and which is suitable to charge fees of decryption and storage of the signal.
According to the present invention, in a recording and reproducing apparatus for selectively recording a digital signal and an analog signal, there are provided a head for digital signal adapted to record a digital signal and a head for analog signal adapted to record an analog signal, and the digital signal head has an azimuth angle which is larger than that of the analog signal head.
Further, the digital signal head has a gap length which is smaller than that of the analog signal head.
Also, in a recording and reproducing apparatus for recording an analog television signal of at least one of PAL, SECAM and NTSC schemes and a digital signal by using rotary heads carried on a cylinder, means for controlling the rotation of the cylinder is provided which controls the rotation of the cylinder such that the rotation number of the cylinder during recording the digital signal is substantially equal to that during recording the analog television signal of the NTSC type.
Further, in a recording and reproducing apparatus for azimuth recording of a digital signal, an analog video signal and an analog audio signal, there are provided a head for digital signal, a head for analog video signal and a head for analog audio signal, and the digital signal head has an azimuth angle which is larger than that of the analog video signal head but is smaller than that of the analog audio signal head.
By performing azimuth recording in which the azimuth angle is different for adjacent tracks, a reproduced signal from an adjacent track can be suppressed. In the reproduced analog signal processing in the VTR, the signal processing is carried out in which a low frequency component of a signal from the adjacent track responsible for reduction of the azimuth effect is suppressed. Since the azimuth angle of the digital signal head is larger than that of the analog signal head, the azimuth effect can be promoted and the low frequency component of the adjacent track signal can be more suppressed during digital signal reproduction than during analog signal reproduction, thereby permitting recording and reproduction in which the error rate is reduced. In addition, by making the azimuth angle of the digital signal head smaller than that of the analog audio signal head, the level of a reproduction signal from the analog audio signal head can be decreased even when a digitally recorded tape is inserted into the analog VTR by mistake, thereby preventing generation of an abnormal sound.
Further, by making the gap length of the digital signal head smaller than that of the analog signal head, high frequency recording and reproducing characteristics for a digital signal can be improved to ensure recording at high density.
Further, by making the rotation number of the cylinder substantially identical for digital signal recording and analog NTSC signal recording, control operation can be facilitated and the rotation number of the cylinder is larger for NTSC recording than for PAL or SECAM recording, thereby increasing the recording rate of a digital signal.
According to another specific form of the present invention, when in a transmitting apparatus a user requests transmission of desired data, a key signal is calculated from a predetermined initial value pursuant to an arithmetic formula corresponding to the user, the desired data requested to be transmitted is encrypted on the basis of the key signal to form main data, and the main data is multiplexed with the initial value and a user identifying code so as to be transmitted.
In a receiving unit, on the other hand, the main data is demultiplexed from the initial value and the user identifying code, reception of the required data requested to be transmitted by this receiving unit is confirmed using the user identifying code, a key signal is generated from the initial value pursuant to the arithmetic formula corresponding to the user, and the required data is decrypted from the main data on the basis of the key signal.
Further, in the receiving unit, after reception of the required data is confirmed, the main data can be multiplexed with the initial value so as to be recorded on a recording unit. Furthermore, when performing recording on the recording unit, the initial value can be converted into a key signal by using the arithmetic formula and thereafter the key signal can be recorded.
In the receiving unit, permission of signal reception by a reception contractor is confirmed using a preset identifying code and thereafter, an initial value delivered out of input signal demultiplexing means is subjected to an arithmetic operation to generate a key signal. For a person who is not the contractor, the correct key signal cannot be obtained from the initial value through the arithmetic operation. For a user who is not given permission of reception from a data distributor, a cipher of main data cannot be decrypted because the main data is encrypted for only a user who is given permission of reception.
Here, the identifying code is a code which indicates that a distributor of a receiving signal allows a reception contractor to receive a signal and which is added to discriminate the present unit from another similar data receiving unit.